Power compensator and method for providing a black start with that compensator

ABSTRACT

A power compensator for an electric power transmission line. The power compensator includes a voltage source converter, a capacitor and an energy storage device. The energy storage device includes a high voltage battery having a short circuit failure mode, a first main switch and second main switch for disconnecting the battery from the capacitor, and a control unit for operating the first and second switch.

TECHNICAL FIELD

The present invention concerns power compensation of a high voltagetransmission line. By a transmission line should be understood aconductor for electric power transmission or distribution line withinthe range of 6 kV and upwards. Especially the invention concerns anapparatus for providing a rapid exchange of electric power on a highvoltage transmission line. The apparatus comprises a voltage sourceconverter (VSC) and an energy storage device. In particular theinvention is related to a black start of a dead network.

BACKGROUND OF THE INVENTION

Within flexible alternating current transmission system (FACTS) aplurality of control apparatus are known. One such FACTS apparatus is astatic compensator (STATCOM). A STATCOM comprises a voltage sourceconverter (VSC) having an ac side connected to the transmission line anda dc side connected to a temporary electric power storage means such ascapacitor means. In a STATCOM the voltage magnitude output is controlledthus resulting in the compensator supplying reactive power or absorbingreactive power from the transmission line. The voltage source convertercomprises at least six self-commutated semiconductor switches, each ofwhich shunted by a reverse parallel connected diode. Since a STATCOMapparatus has no active power source it can only compensate for reactivepower.

From U.S. Pat. No. 6,747,370 (Abe) a power compensation system using aconverter connected to a high temperature secondary battery ispreviously known. The object of the compensation system is to provide aneconomical, high-temperature secondary battery based energy storage,which have a peak shaving function, a load leveling function and aquality stabilizing function. The known system comprises an electricpower supply system, an electric load and an electric energy storagesystem including a high temperature secondary battery and a powerconversion system. The battery is a sodium sulfur battery.

The system is arranged at an end of an electric power line. The load isa factory which under normal operating condition is provided withelectric power supply from the power line. In case of power supplyfailure a high speed switch disconnects the power line and electricpower is instead provided from the secondary battery. At the same time aback up generator is started. The known system having a sodium sulfurbattery indicates that the power compensating system provides low powerduring a long time.

In one mode of operation the battery is providing extra energy to thefactory during daytime while being recharged during night. In order tosupply a factory with uninterruptible power there are arranged tenparallel connected battery units of 1280 V, each having a converter of500 kW. In a further embodiment ten battery units are parallel connectedin series with a 5 MW converter. In this embodiment a group of sparebatteries is arranged for use with the high temperature battery circuit.In the event of a battery unit having a failure the failed unit isdisconnected and the spare group is connected in parallel with thecircuit.

Restoring power after a wide-area power outage can be difficult. Aplurality of power stations needs to be brought back on-line. Normally,this is done with the help of power from the rest of the grid. In theabsence of grid power, a so-called black start needs to be performed tobootstrap the power grid into operation.

To provide a black start, some power stations are typically equippedwith small diesel generators which can be used to start largergenerators (of several megawatts capacity), which in turn can be used tostart the main power station generators. Generating plants using steamturbines require station service power of up to 10% of their capacityfor boiler feedwater pumps, boiler forced-draft combustion air blowers,and for fuel preparation. It is, however, uneconomic to provide such alarge standby capacity at each station, so black-start power must beprovided over the electrical transmission network from other stations.

A typical sequence (based on a real scenario) might be as follows:

-   -   A battery starts a small diesel generator installed in a        hydroelectric generating station.    -   The power from the diesel generator is used to bring the        hydroelectric generating station into operation.    -   Key transmission lines between the hydro station and other areas        are energized.    -   The power from the hydro dam is used to start one of the        coal-fired base load plants.    -   The power from the base load plant is used to restart all of the        other power plants in the system including the nuclear power        plants.    -   Power is finally re-applied to the general electricity        distribution network and sent to the consumers.

To restore the power after an outage is not an easy process. Smalldisturbances are continually occurring while the system is weak andfragile during the restoration process, and the grid will experiencedifferent conditions from a dead network over a variety of weak networkconditions to a normal strong AC network. In order to maintain thefrequency and voltage stability during the restoring process, an overallcoordinated system restoration plan is necessary. Hence a black startcontains a first stage where the network is energized. In this stage thefrequency and a phase angle must be established. In a second stage whichis the recovery stage the network is unstable and very vulnerable. Inthis stage a certain slack must be provided by which is meant thatenergy must be provided to the network during part of the time whileduring another part of time energy must be transferred away from thenetwork.

SUMMARY OF THE INVENTION

An exemplary object of the present invention is to seek ways to improvea black start of a power network.

This object is achieved according to the invention by a powercompensator characterized by the features in the independent claim 1 orby a method characterized by the steps in the independent claim 8.Preferred embodiments are described in the dependent claims.

According to the invention a power compensator comprising a voltagesource converter (VSC), capacitor means and a powerful electric energysource connected in parallel thereto is providing both energizingcapacity to the network and recovery capacity of the network by beingcapable of both delivering electric energy and consuming, by loading thebattery, electric energy from the network. To influence the black startprocess there must be provided to the dead network an input power ofsufficient strength and with sufficient duration. Such power source isaccording to the invention provided by a powerful battery meanscomprising a short circuit failure mode. By short circuit failure modeshould be understood that in case of an interior failure of the energystorage device the electric circuit will be kept closed. The shortcircuit failure mode may be effected by the inner performance of thebattery cell. It may also be effected by a controllable switch making aparallel loop with the battery cell.

To start up a voltage source converter connected to a dead network isnot an easy task. Normally the VSC is connected to the network by atransformer. In the transformer there are separate winding to provideauxiliary power to the VSC and its equipment. There are also embodimentswhere auxiliary power is provided by a separate transformer connected tothe power line. In a dead network no auxiliary power can be providedfrom these solutions. Hence a first power source must be arrangedinternally to provide auxiliary power for the control means.

In a fault situation the exchange of power with the network is stopped.A switch on both electrodes connecting the battery to the capacitor isopened. This means that in a total outrage of the network there is stillpower contained in the battery means of the apparatus. In performing ablack start the capacitor means is slowly energized from the batterymeans by connecting a resistor means in series with both electrodes ofthe battery means. As soon as the capacitor means is energized switchingof the converter may begin. This results in an ac power to the network.This will also provide auxiliary power to the converter from atransformer on the ac side.

By providing power to the network in this way other generation units onthe network will be provided with auxiliary power to come into powerproduction. Hence in a few moments of time a plurality of generationsunits may be in production. In the restoration phase more and more powerproducers and consumers will be connected. However, the connections ofthese producers and consumers may occur in an uncontrolled way. Thismeans that in one moment of time there will be an overproduction whilein a second moment there will be an overconsumption of power. Thenetwork therefore needs to be balanced. According to the invention thisis achieved by the power compensator. Hence when the network experiencean overproduction of power the battery means of the power compensator ischarged and when the network experience an overconsumption the batterymeans is providing the power needed to keep the network in balance.

The power supplied VSC may act very rapidly since it has no inertia. Onthe other hand the VSC must comprise safety arrangement not to beoverloaded. The series connected extinguishable semiconductors of theconverter valve cannot handle too high a current.

Since the energy storage device must be capable of exchanging energy atall times there must be arranged for redundancy in case of a batteryfailure. Batteries having an open circuit failure mode must therefore beconnected in parallel. Batteries having a short circuit failure mode maybe connected in series thus reaching much higher voltage levels. In anembodiment of the invention the energy storage device comprises a highvoltage battery containing a plurality of battery cells, each having ashort circuit failure mode. A plurality of such batteries connected inseries will always provide a closed circuit and thus be capable ofproviding electric energy even with a battery cell failure. A pluralityof batteries connected in series will also be capable of providingenergy at high voltage in the range of 6 kV and above.

According to an embodiment of the invention the battery comprises a hightemperature battery containing a plurality of sodium/metal chloridebattery cells having an operating temperature in the range of 270-340°C. A battery unit comprises a heat insulated box containing a pluralityof series connected battery cells. The battery unit has two terminalscomprising an electric circuit in the range of 1500 V. Connecting foursuch battery units in series will thus reach a voltage level of 6 kV.The battery unit comprises a local pipe loop for housing a heat transfermedium in the form of a fluid. The fluid may be a liquid medium as wellas a gaseous medium.

A criteria for the function of the battery, e.g. to be able to store andrelease electric energy, is that the temperature inside the battery cellis kept between 270 and 340° C. At operation mode such as when thebattery is being charged or discharged heat is generated within thebattery. At idling mode, however, no heat is generated inside thebattery. Thus at the idling mode heat has to be provided from outsidethe battery.

In an embodiment of the invention the power compensator comprises atemperature monitor for maintaining the operation temperature of thebattery unit. Thus the temperature monitor is providing heat during theidling mode. The temperature monitor contains a pipe network forproviding a flow of the heat transfer medium through the battery units.The pipe network comprises a main pipe loop and at least one fluidmoving unit, such as a fan or a pump. The pipe network includes thelocal pipe loop of each battery unit and provides a passageway for theheat transfer medium. The heat comprised in the heat transfer medium istransferred to the battery cells by convection.

According to an embodiment of the invention the local pipe loopcomprises a first end for receiving a stream of a gaseous medium, and asecond end for exhausting the gaseous medium. In an embodiment thegaseous medium comprises preferably air. Further the main pipe loopcomprises an upstream side for providing hot air and a downstream sidefor receiving disposed air. Each first end of each local pipe loop isconnected to the upstream side of the main pipe loop. Each second end ofthe each local pipe loop is connected to the downstream side of the mainpipe loop. All connections between the main pipe loop and each localpipe loop comprises a connection pipe. The main loop comprises at leastone fan and a heat providing means. In an embodiment of the inventionthe main pipe loop is grounded and thus exhibits the ground potential.Each local pipe loop exhibit the same potential as the battery unithousing the local pipe loop. In a further embodiment each connectionpipe comprises a tube of a heat resisting and electric insulatingmaterial, such as a ceramic material.

In yet a further embodiment of the invention the temperature monitor isalso during the operation mode of the battery unit providing a cooledair for disposal of heat generated from the battery cells.

According to an embodiment of the invention the power compensator systemcomprises a system for controlling the performance and the action of thepower compensator. The control system contains a charge monitor formaintaining the charge and discharge respectively of the energy storagedevice. Since the charging and discharging behavior of a sodium/metalchloride battery is complicated the state of charge (SOC) of the batterycannot be measured but must be estimated. Also the current of thebattery cannot be measured with a sufficient accuracy. The chargemonitor therefore comprises a SOC-module for estimating and predictingthe state of charge of the battery.

A sodium/metal chloride battery cell comprises an electrolyte containedin a thin barrier of a ceramic material. Outside the barrier the batterycell comprises sodium being a first electrode. The second electrodecomprises a pair of nickel plated copper electrodes to which isconnected a metallic structure spreading into the electrolyte. When thebattery is charged or discharged a reaction front is propagatinginwardly from the ceramic barrier. Thus both the charging anddischarging is propagating in the same direction and starting from theceramic barrier. Resulting from a plurality of charging and dischargingcycles there may be left inside the battery cell a plurality of areasdefining power capacity areas and non-power capacity areas. Hence theSOC-module must be capable to sum only the areas which represent powercapacity.

The SOC-module comprises a virtual model of the battery. The virtualmodel contains a plurality of model parts representing specificrelations of parameters and input values. Thus the virtual modelcomprises a measurement part model containing the relation betweenvoltage, current, temperature and other parameters. Further the virtualmodel contains a part model for estimating the actual SOC valuecontaining memory means for historic data. The virtual model alsocontains a part model for predicting a future SOC-value containing acalculating model. Another part model is relating to historic data suchas charging events, discharging events, recovery data and such.

The main objective of the virtual model is to produce a SOC-value whichrepresents the remaining capacity of the battery. The SOC-value may bepresented as a percentage value of full capacity of the battery. Anotheraim for maintenance of the battery comprises charge and discharge of thebattery such that overcharges or undercharged never occurs and such thatthe battery temperature is always kept within the allowable range.

By using the virtual model of the battery the SOC-module predicts alsothe SOC-value at a later point in time dependent on the power profileand duration. While using the capacity of the battery in a powercompensation situation the predicted SOC-value will tell if there issufficient capacity for a predetermined mission. If for instance thereis a power shortage in the transmission line the predicted SOC-valuewill tell if the capacity of the battery is sufficient for providingenergy during a given period of time. This may happen after a power linefailure and before power is provided again by other sources, such asstart up period of a generator. If there is an excess of generated poweron the transmission line, for instance due to a fault, the predictedSOC-value will instantly tell if the battery is capable to receive powerfrom the transmission line. Hence the power compensator according to theinvention is capable of both providing energy and receiving energy fromthe transmission line in a short time perspective, such as milliseconds,as well as in a longer time perspective, such as minutes.

In an embodiment of the invention the control system comprises aplurality of sensors for sensing voltage, current, temperature and otherparameters. For electric power supply to these sensors the systemcomprises a power supply unit on each battery unit. The power supplyunit is galvanic isolated from earth and comprises the same potential asthe battery unit. The power supply may comprise a fuel cell, a solarcell, a thermo-electric element such as a peltier element and others. Inan embodiment the power supply unit comprises battery means. For sendingthe information to the control system each sensor may communicate byhelp of a wireless system or an optical fiber. Each battery may alsocomprise a central communication device for communication ofinformation.

According to an embodiment of the invention there is arranged on eachgalvanically isolated battery unit a communication module. The modulecomprises radio communication means, power supply and a plurality ofsensing transducers. Also the communication module is galvanicallyisolated and thus achieving the same potential as the battery unit. Themodule may communicate within a wireless local area network, such as aWLAN or a Bluetooth network. The sensed values, such as voltage, currentand temperature are preferably transmitted in digital form. To savepower consumption the communication is arranged in short part of a timeperiod. Thus the communication means need only be electrified during asmall percentage of time. The communication may preferable take placewithin the 2 GHz band. The power supply comprises in one embodiment aback up battery and electric energy providing means. Such energy meansmay comprise any kind of generator configuration as well as a solarcell, peltier element, a fuel cell or other means.

According to a further embodiment of the invention a SOC-value isestimated by current value provide from multiple parallel calculationswith the help of the virtual battery model. A first value of the voltageover the battery unit is calculated from the measured current. Inparallel the voltage is calculated with a plurality of chosen currentvalues, each deviating a small amount from the measured current value.Each such calculated voltage value is compared with the actual measuredvoltage value. When a close match between the calculated voltage valueand the measured voltage value is achieved the input current value forthe matching calculation is chosen as the actual current value.

In a first aspect of the invention the object is achieved by a powercompensator for an electric power transmission line, the compensatorcomprising a voltage source converter and an energy storage device,wherein the energy storage device comprises a high voltage battery meanshaving a short circuit failure mode. In a further embodiment the energystorage device comprises a high energy, high temperature sodium/metalchloride battery. In yet a further embodiment the power compensatorfurther comprises a temperature monitor for keeping the temperaturewithin the operation range of the battery means. In still a furtherembodiment the power compensator further comprises a control systemcontaining a charging monitor for providing a state of chargingestimation of the battery means. In still a further embodiment thecharging monitor comprises a SOC-module containing a virtual model ofthe battery for providing a parallel calculation of the current flow ofthe battery.

In a second aspect of the invention the objects are achieved by a methodfor providing a power compensation of an electric power transmissionline, wherein the method comprises, forming an energy storage device ofa battery means containing a plurality of series connected battery unitshaving a short circuit failure mode for achieving a voltage in the rangeof 6 kV and above, providing in a first mode of operation electricenergy from the battery units to the transmission line, and receivingduring a second mode of operation electric energy from the transmissionline to the battery units. In a further embodiment the method comprisesthat each operation mode comprises an estimation of the state of chargeof the battery means.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become moreapparent to a person skilled in the art from the following detaileddescription in conjunction with the appended drawings in which:

FIG. 1 is a principal circuit of a power compensator according theinvention,

FIG. 2 is a side elevation of an energy storage device comprising aplurality if battery units according to the invention,

FIG. 3 is a principal layout of a power compensator including atemperature monitor and a charge monitor,

FIG. 4 is side elevation of an energy storage device and a temperaturemonitor,

FIG. 5 is a further embodiment of the temperature monitor, and

FIG. 6 is a yet further embodiment of the invention for black startcapacity.

DESCRIPTION OF PREFERRED EMBODIMENTS

A principal circuit of a power compensator 1 connected via a transformer2 to an electric power transmission line is shown in FIG. 1. The powercompensator comprises a voltage source converter 4, a capacitor means 6and an energy storage device 5. The voltage source converter comprisesat least six selfcommutated semiconductor switches, each of which isshunted by a reverse parallel connected diode. The voltage sourceconverter has an ac side connected to the transformer and a dc sideconnected to the capacitor means and the energy storage device.

The energy storage device comprises a plurality of series connectedbattery units 7. In the embodiment shown in FIG. 2 four battery units 7a-7 d are arranged in a rack 8. Each battery unit has a positiveterminal 9 and a negative terminal 10. In the embodiment shown eachbattery unit has a voltage of 1500 volts thus the energy storage devicecontaining four batteries connected in series has a voltage level of 6kV. However there may also be many more batteries in series resulting ina much higher voltage level.

The energy storage device comprises high energy, high temperaturebatteries containing sodium/metal chloride battery cells having anoperating temperature in the range of 270-340° C. Each battery unitcomprises a heat insulated box containing a plurality of seriesconnected battery cells. In operation such as charging or dischargingthe batteries produce heat. At the idling mode heat from outside thebattery must be provided for keeping the operational temperatureconditions. The battery unit therefore contains a local pipe loop havinga first opening 11 for receiving a stream of a gaseous medium, and asecond opening 12 for exhausting the gaseous medium.

A sodium/metal chloride battery cell comprises an electrolyte containedin a thin barrier of a ceramic material. When the battery is charged ordischarged a reaction front is propagating inwardly from the ceramicbarrier. Thus both the charging and discharging is propagating in thesame direction and starting from the ceramic barrier. Resulting from aplurality of charging and discharging cycles there may be left insidethe battery cell a plurality of areas defining power capacity areas andnon-power capacity areas.

in further embodiment of the invention is shown in FIG. 3. In thisembodiment the power compensator 1 comprises not only a voltage sourceconverter 4 and an energy storage device 5 but also a temperaturemonitor 13 and a control system 14 containing a plurality of sensormeans 39, computer means 41 and a charge monitor 15. The charge monitorcomprises a module 16 for estimating the state of charge of the battery.The temperature monitor 13 comprises a pipe network for housing a heattransfer medium. The pipe network comprises a main pipe loop 17, a localloop 18 located in each battery unit and a plurality of connection pipes19 connecting the main loop with the local loops. The temperaturemonitor contains at least one heat providing means and a fluid movingunit for circulating the heat transfer medium in the pipe network. Henceby circulating the heat transfer medium through each battery heat isprovided to the batteries by convection. In the embodiment shown theheat transfer medium comprises air and the fluid moving unit comprises afan.

In FIG. 4 the temperature monitor 13 is schematically divided into amain pipe loop 17 and a common local pipe loop 18. In this embodimentthe local pipe loop exhibits a high voltage potential while the mainloop exhibits a ground potential. The connection pipes which connect themain pipe loop and the local pipe loop must not only exhibit an electricinsulation but also withstand a fluid medium having a temperature ofapproximately 300° C. The main loop in this embodiment comprises aseparate fan 20 and a pipe part 21 for each battery unit. Each pipe partcomprises a heat providing element 22 for heat delivery to the batteryunit. The heat delivery unit may comprise a resistive element forconnection to a low voltage electric power source.

A further development of a temperature monitor is shown in FIG. 5. Inthis embodiment the main loop of the temperature monitor furthercomprises a common heating system 23 including a heater 22 and a commonfan 20. According to this embodiment there is also provided for coolingof the battery units. Thus there is arranged a cooling loop 25 with acooler 26 and a common cooling fan 27. The provision of cooling orheating may be chosen by a switching valve 28. Also in the embodimentshown the heating system comprises an extension loop passing through aheat storage device 31. Further the system comprises a second loop 29passing through a heat exchanger 32 for heat exchange with a secondfluid system 33 which may comprise cooling water from the voltage sourceconverter valves. The heating system also comprises a an extension looppassing through a second heat exchanger 35 for heat exchange with secondheating system 34 which may be a heating system for a building.

A principal circuit of a power compensator 1 for black start capabilityis shown in FIG. 6. Using the same references as in FIG. 1 the powercompensator is connected via a transformer 2 to an electric powertransmission line. The power compensator comprises a voltage sourceconverter 4, a capacitor means 6 and an energy storage device 5. In theembodiment shown the energy storage supply comprises battery means. Thevoltage source converter has an ac side connected to the transformer anda dc side connected to the capacitor means and the energy storagedevice. The power compensator comprises a first and second main switch40 a, 40 b for disconnecting the battery means. In parallel with themain switches there are arranges a first and second parallel pathcontaining a resistor means 41 a, 41 b and a secondary switch 42 a, 42b. The power compensator also comprises a control means 44 forcontrolling the switches. Normally the control means receives auxiliarypower from a separate winding 43 of the transformer.

In case of a power outrage of the network the control means alsocomprises battery means and computer means for control of the switchesduring a dead network situation.

Although favorable the scope of the invention must not be limited by theembodiments presented but contain also embodiments obvious to a personskilled in the art.

1. A power compensator for an electric power transmission line,comprising: a voltage source converter having an ac side connected tothe transmission line and a dc side connected to a capacitor; and anenergy storage device, wherein the power compensator is configured toprovide a black start of a dead transmission line, the energy storagedevice is configured to energize the capacitor and comprises a highvoltage battery having a short circuit failure mode, a first main switchand second main switch for disconnecting the battery from the capacitor,and a control unit for operating the first and second switch, andwherein the power compensator is configured to perform said black startby energizing the capacitor from the battery and when the capacitor hasbeen energized start switching the voltage source converter resulting ina ac power being provided to the transmission line.
 2. The powercompensator according to claim 1, wherein further comprising: a firstbranch comprising a first resistor and a first secondary switch, whereinthe first branch connected in parallel with the first main switch. 3.The power compensator according to claim 1, further comprising: a secondbranch comprising a second resistor and a second secondary switch,wherein the second branch is connected in parallel with the second mainswitch.
 4. The power compensator according to claim 1, wherein theenergy storage device comprises a high energy, high temperaturesodium/metal chloride battery.
 5. The power compensator according toclaim 1, further comprising: a temperature monitor for keeping thetemperature within the operation range of the battery.
 6. The powercompensator according to claim 1, further comprising: a control systemcomprising a charging monitor for providing a state of chargingestimation of the battery.
 7. The power compensator according to claim5, wherein the charging monitor comprises a SOC-module comprising avirtual model of the battery for providing a parallel calculation of thecurrent flow of the battery.
 8. A method for providing a black start ofa dead network comprising a voltage source converter, a capacitor and anenergy storage device, the method comprising: energizing the capacitorfrom the energy storage device, starting switching the voltage sourceconverter, and controlling a power flow of the power compensator to andfrom the network in dependence on a balance of power producers and powerconsumers connected to the network.
 9. The method according to claim 8,wherein the energizing comprises forming a current path comprising aresistor in order to decrease a current flow between the capacitor andthe energy storage device.
 10. A computer program product, comprising: acomputer readable medium; and computer program instructions recorded onthe computer readable medium and executable by a processor to carry outa method for providing a black start of a dead network comprising avoltage source converter, a capacitor and an energy storage device, themethod comprising energizing the capacitor from the energy storagedevice, starting switching the voltage source converter, and controllinga power flow of the power compensator to and from the network independence on a balance of power producers and power consumers connectedto the network.
 11. The computer program product according to claim 10,wherein the computer program instructions are further for providing thecomputer program instructions at least in part over a network.
 12. Thecomputer program product according to claim 10, wherein the computerprogram instructions are further for providing the computer programinstructions at least in part over the internet.